chore(.travis.osx.yml): update .travis.osx.yml

 - Use gcc-4.9.2
 - Try to resolve opam issue:
   - https://github.com/Homebrew/homebrew/issues/33930
   - https://github.com/ocaml/opam/issues/1853
 - Bump OSX version
chore(.travis.osx.yml): update .travis.osx.yml